The present invention relates to apparatus for transforming an air-fibre dispersion stream and is applicable in pulp-and-paper, textile and building industries for production of various grades of paper, nonwoven fabrics, felts and other fibrous materials homogeneous in structure.
The transforming of the air-fibre stream discussed herein is assumed to be such a change which results in increasing of a stream cross-sectional area and consequent reduction of an average stream velocity, a uniform distribution of a velocity field of the stream in its cross-section, a generation of a fine-scale turbulence in the air-fibre mixture, a homogeneous fibre distribution throughout the stream and an increasing of a dispersity value of the air-fibre suspension while the stream is being transformed.
The distribution of the velocity field of the air-fibre stream, as applied to the apparatus of the invention, is assumed to be uniform in the stream cross-section if the coefficient of variation of random local velocities is not over 17 percent.
The dispersity value of the air-fibre stream is assumed to be a ratio of a volume of discrete fibers or small fibrous aggregates to a volume of an individual fibre of mode-length, i.e. the length which predominates in the fibre length distribution.
Let the dispersity value of the air-fibre mixture be no more than 60. The volume of the individual mode-length fibre is obtainable from the direct measurement of the fibre sizes. The fibrous aggregate volume may be estimated by the use of high speed filming.
The homogeneous fibre distribution in the air stream assumes the fibre concentration in each individual stream volume to have little or no fluctuations.
The degree of uniformity of the concentration distribution of the fibres in the air stream can be indirectly determined by an optical method, using a light beam measuring 0.5.times.0.5 mm in its cross-section.
Fluctuations of the fibre concentration in the air stream change an intensity of the light passing through the stream and striking a photocell. The light intensity fluctuations result in changing of a photoelectric current developed in the cell. The recording means enables anyone to calculate the coefficient of variation of the light intensity fluctuations, thus variations of the fibre concentration in the stream are indirectly represented.
With the distribution of the fibre concentration uniform, the coefficient of variation, determined by the aforementioned method, is in the range from 18 to 25 percent.
The transforming of the air-fibre mixture is quite important, because it enables fibrous material having homogeneous structure, predominantly paper, i.e. a material with uniform physic-chemical properties, to be produced in a forming aerodynamic unit.
For example, when filter paper is being produced, the transforming of the air-fibre stream provides the uniformity of such properties as porosity, tensile strength, shearing strength, coefficient of extensibility etc., i.e. the transforming of the stream effects the quality of the finished materials.
In combination with the forming aerodynamic unit of the known design the apparatus of the invention may be used in various machines and means where fibrous material homogeneous in structure is produced.
In order to make a web of the fibrous homogeneous material various kinds of fibre are used: native, including chemical wood, chemical, non-organic, and preferably fibres of 0.5 to 15 mm long. For example, when various grades of paper or nonwoven fabrics are being produced, mechanical wood, sulphate and sulphite cellulose, lint cotton, flax, asbestos and other kinds of fibre are used.
The kind of fibre generally used depends on properties of the material to be produced therefrom. So, asbestos fibre provides the material with high thermal and electrical insulation properties, polypropylene fibre permits the material being produced to be alcali resistant, while cotton and dacron provide biological safety for the finished product.
The fibrous materials with various properties, e.g. the aforementioned, are applicable in aircraft, electrotechnical, metallurgy and other industries, and are of great importance, enabling many technical problems to be carried out.
The transforming of the air-fibre dispersion stream, enhance production of fibrous material homogeneous in structure, is known to have the following requirements:
The uniform distribution of the velocity field in the cross-section of the air-fibre steam is to be achieved; this is necessary to produce the fibrous material homogeneous in structure edgewise by the aerodynamic forming unit. The velocity field being elongated, for example, along its longitudinal axis, results in the manufacture of the material having a "hill-shaped" fibre distribution edgewise and having a strongly marked maximum in its middle. The fibre distribution throughout a thickness of the material should be similar to the abovementioned.
The velocity of the air-fibre mixture is to be reduced in order to provide for the production of the fibrous material while supplying the transformed air-fibre stream parallel to a wire screen of the forming unit. The stream velocity equal to a rate of the travelling wire prevents the latter from disturbing the transformed stream, consequently resulting in a non-homogeneity of the material in structure.
The fine-scale turbulence of the flow to be generated that, as well known to those skilled in the disperse system arts, does not result in a local fibre flocculation, the latter having a negative effect on the homogeneity of the fibrous material.
The dispersity value of fibrous solids in the air-fibre mixture is to be increased by providing a homogeneous distribution over an entire stream volume, i.e. a homogeneous fibre dispersion in the air stream is to take place. This is an important condition of the production of a fibrous material homogeneous is structure.
Apparatus for the distributing of the air-fibre mixture are known, these apparatus comprising means for expanding the stream before it is supplied onto the wire of the forming unit. From a disperser chamber the air-fibre stream through a discharging pipe generally enters a diffuser, having diverging side walls and converging frontal walls. In the diffuser the stream is expanded edgewise, without reducing the velocity, thus not resulting in an elongating of the stream velocity field; this elongating is usually peculiar to diffusers. Owing to such a design, the uniform distribution of velocities of the transformed air-fibre stream is achieved.
However, the apparatus described does not provide the required homogeneous distribution of the fibres with high dispersity value and hence it is not applicable for the production of homogeneous material, when the fibre dispersity value is high.
Another type of stream transforming apparatus represents a diffuser, having diverging side and frontal walls and rotating rollers, arranged therein, the latter comprising teeth.
The air-fibre stream supplied to the diffuser is subjected to a treatment of said rollers, thus resulting in the uniform distribution of the flow velocity field. The velocity smoothing is attended with mechanical agitating of the air bulk and separating of large aggregates into small fibrous solids.
This type of apparatus also does not provide all parameters to fulfill the conditions which are necessary for producing fibrous homogeneous material, because the fine-scale turbulence of the stream and the homogeneous distribution of fibres in the air are not achieved.
One more apparatus for stream transforming represents a diffuser with a faired-form body disposed therein.
The air-fibre stream, supplied to the apparatus through an inlet pipe, interacts with said body and uniformly distributes all over the diffuser cross-section thus smoothing the velocity field of the stream. The latter is expanded vertically and edgewise horizontally and as a result the flow velocity is reduced.
In apparatus of such type the desired transforming of the air-fibre stream also does not take place.
Yet another modified apparatus for the transforming of the air-fibre stream is known. It comprises a diffuser with diverging side walls and an inlet opening communicating with the stream supplying pipe-line. An outlet opening of the diffuser is connected with a rectangular upright chamber. Several faired airfoil-form bodies are arranged in the chamber with their planes parallel to the chamber side walls, the upper portion of each body disposed inside the diffuser. The air-fibre suspension supplied to the diffuser is impacted against the faired-form bodies and distributed edgewise over the rectangular chamber. The flow velocity is reduced, and the uniform distribution of the velocity field is attained.
This apparatus cannot provide the fine-scale turbulence of the stream, because of its interaction with the faired airfoil form bodies. This interaction generates a large-scale turbulence, thus hindering the homogeneous fibre distribution required to be achieved.
No one of the known stream transforming apparatus can provide the desired degree of transforming of the stream of high fibre concentration, especially, when the air-fibre flow is assumed to be fluidized, i.e. when the volume concentration of fibres is equal to 0.3.